Typically, in such a digital image processing system, a compression of the digital image signal band is performed in order to improve an image signal transferring efficiency. Such a compression of the digital image signal is substantially executed in such a way that an image signal input in the time domain is transformed into the frequency domain. As a result, most of the image energies are condensed into some of the transform coefficients of each N.times.N block (where N is a positive integer). The transform coefficients of relatively low energy are converted to zero ("0") by comparing the coefficients with a predetermined threshold value so as to extend the number of "0" valued coefficients as much as possible. The coefficients are then converted into a serial data stream by way of a fixed scanning method such as a zigzag scanning method, in which a constant scanning slope of "1" is used. That is, the number of zero-valued transform coefficients (i.e., runlength) is counted to transfer a column (runlength, amplitude) of the image data. An end of block (EOB) bit follows the last non-zero valued transform coefficient in the block. The zero-valued coefficients following the last non-zero valued transform coefficient are not considered as practical data and only the number of zero-valued coefficients is transferred to establish the desired data compression. The data is compressed from its original form because only the coefficients having a non-zero value are transferred. The scanning of the image data to form the data stream is effected by using a fixed slope zigzag scanning method as previously described. FIG. 1 illustrates an example of the zigzag scanning method, in which N is set to eight transform coefficients and numerals in the block denote the scanning order.
In FIG. 1, the transform coefficients are partially distributed in a horizontal, vertical or diagonal direction in accordance with the input image pattern. For this reason, when the image data is scanned by the fixed-slope zigzag scanning method, the EOB bit located after the last non-zero valued coefficient is not produced as quickly as possible. Specifically, the fixed slope zigzag scanning method will produce the EOB bit as quickly as possible only for one particular energy distribution of transform coefficients, the distribution in which the non-zero valued coefficients are located in the shaded area of FIG. 4C and the zero-valued coefficients are located in the non-shaded area. This is because this particular input distribution corresponds exactly with the zigzag scanning method. For other types of input patterns, (e.g. FIGS. 4A, 4B, 4D and 4E), however, the conventional zigzag scanning method which always uses a slope of "1" will not correspond exactly with the input patterns. This causes a problem in that the data compressing efficiency for these other input patterns is likely to be limited-because the number of zero valued coefficients which follows the EOB bit is also transferred to compress the image data. The transferring efficiency is maximum when only the non-zero valued coefficients are transferred.